The present invention relates to threaded fasteners and more particularly to a self-tapping screw and a method and apparatus for making same.
A wide variety of self-tapping screw forms are presently available. Self-tapping screws may be of the thread forming type, thread cutting type or the metallic drive type. The thread forming type self-tapping screws typically include a continuous thread and a tapered point. These screws are designed to plastically displace material adjacent to a pilot hole in order to form the threads which mate with the screw. Thread cutting type tapping screws include cutting edges and cavities or flutes. These types of screws physically remove material or cut the material adjacent the pilot hole to form the mating threads. Metallic drive screws are typically forced into the material to be joined by pressure. These types of screws are typically used where permanent fastenings are desired.
The various forms of tapping screws presently available have been standardized. The particular types of thread forms are typically designated ASA standards. For example, an ASA Type AB tapping screw form includes a shank portion having a spaced, continuous thread and which terminates in a gimlet point. A Type AB thread form is typically employed to join sheet metal, resin and impregnated plywood, wood and asbestos compositions. The gimlet point permits it to be used in pierced or punched holes where the sharp point may be necessary for starting of the thread forming. Another ASA type is designated Type B. This type of tapping screw form includes a cylindrical shank which terminates in a tapered, blunt-point. A spaced-thread screw is continuous on the shank and extends onto the blunt-point. The Type B thread form is typically designated for use in heavy-gauge sheet metal and nonferrous castings.
In the fabricating of tapping screws, a screw blank is typically fabricated from a cut length of wire stock. Typically, the stock is fabricated from a cold heading quality, annealed and processed low carbon steel. The cut length of stock is disposed within the bore of a male heading die and the lower end of the stock contacts a knock-out pin. A heading die or punch is moved into engagement with the end of the stock extending from the female die bore and deforms the stock to head the blank. In the fabrication of self-tapping screw forms wherein the lower end of the shank portion of the screw is tapered, the bore of the female heading die is formed with a taper so that the resulting blank includes a head and an integral, depending cylindrical shank portion which terminates in a tapered point having a circular cross section.
Typically, the screw blank will have the thread cut therein by a thread rolling machine. Thread rolling machines include complementary, relatively moving dies. Each die includes a face having a plurality of cutting ridges formed therein. The cutting ridges extend downwardly at an angle relative to the direction of die movement. The angle corresponds to the helix angle of the formed screw thread. The ridges are separated by grooves a distance which equals the pitch of the thread form. One of the dies is held stationary and the other die is reciprocated by the machine. The blank is received between the dies, is rolled between the relatively moving dies and the ridges cut the blank to form the thread formed therein. An example of a machine of this type may be found in U.S. Pat. No. 3,117,473, entitled THREAD ROLLING MACHINE and issued on Jan. 14, 1964 to R. D. Morton et al.
In the manufacture of self-tapping screws, especially those of the ASA Type B thread form, problems have been experienced with the service life of the heading dies, the service life of the thread cutting dies and with the attainment of good quality, continuous, crisp and sharp threads along the tapered portion of the screw. For example, formation of a tapered end in the shank of the screw blank results in excessive or high stresses being exerted on the female heading die. The female die at its tapered base will fracture due to the stress concentrations. Further, various methods used to roll the thread into the blanks have not resulted in formation of a good quality thread in the area of the taper. To increase thread quality, it has been proposed to employ dies having a straight, longitudinal section and an outwardly sloped or tapered section which will follow the taper on the shank of the blank in order to cut the threads. An example of dies which are configured to mate with the tapered portion on the shank of a blank may be found in U.S. Pat. No. 2,314,391, entitled SCREW AND ITS METHOD OF MANUFACTURE and issued on Mar. 23, 1943 to W. A. DeVellier. Another example of the prior approaches to the fabrication of self-tapping screws may be found in U.S. Pat. No. 3,772,720, entitled METHOD FOR MAKING A THREAD FORMING MEMBER and issued on Nov. 20, 1973, to Yoshio Yamamoto. Tapering of the thread cutting dies does not correct the problems which have heretofore been experienced with respect to failure of the heading dies employed to form the tapered shank screw blank.
If the screw blank is fabricated in a header die with a constant diameter shank portion to eliminate the fracture problems in the header dies, proper rolling of the threads and/or reliable service life of the dies will not be obtained. In order to properly form the tapered point on the self-tapping screws, the threading dies must define an outwardly sloping or tapered section which deforms the lower end of the shank in order to form the point and cut the threads therein. If the shank is of constant diameter in cross section, high stress concentrations occur at the sloped portions of the dies which can result in fracture and reduced service life for the dies.